With an increasing demand for an increase in capacity and speed of data processing, it is pointed out that future computers and integrated circuits have limits in the increase in speed, power saving, and downsizing. As a breakthrough to the limits, there is a need for realization of a device/system in which the optical wiring and an electronic circuit are combined, as well as a need for development of a technical field that uses a new electronic phenomenon of a semiconductor.
Under these circumstances, Si photonics that uses a silicon microprocessing technology to form an optical circuit on a silicon on insulator (SOI) substrate gains attention, and development of an optical interconnection technology that is characterized by the increase in speed, reduction in power consumption, and downsizing has been made actively.
Integration of an optical modulator, a photodetector, and an optical waveguide is needed to form the optical circuit on the Si, and the biggest problem is how to direct light, as a signal, to the waveguide formed on the Si. Although the Si itself does not have an emission mechanism, there is a recent report that laser operation is realized by collectively forming emission materials on the Si. It has been reported that, for example, a Ge laser is formed on the Si (refer to, for example, Optical Fiber Communication Conference (OFC2012), PDP5A), and that a quantum dot laser whose wavelength is in 1.3 μm band is formed on the Si substrate (refer to, for example, Optics Express, 19(12), 1138141386 (2011)).
However, these are still under development, and improvements in characteristics of a threshold current, optical output and the like are needed. A method of inputting light from the outside into the optical circuit and the optical waveguide that are formed on the Si is also examined, other than the method of building the emission mechanism onto the Si substrate. A method of reducing only the width of the waveguide is known as the simplest method, in which low-loss optical coupling is realized by allowing the spot size of a semiconductor laser and the spot size of the spot-size converter provided on the Si side to be nearly uniform.
There are several known methods, other than the above-described methods, to cope with some cases in which connection of the optical fiber to the outside is needed. For example, it is suggested that a V groove is formed on a planar lightwave circuit (PLC) side, the optical fiber is fixed into this V groove, and light is inputted into the optical circuit (refer to Japanese Patent Application Laid-open No. H08-313756). Although this suggestion does not have the description about the spot-size converter on the optical circuit side, which is needed in actuality, it is assumed that the one based on a tapered waveguide that reduces the width of the waveguide is employed.
It is also suggested that fiber is surface-coupled by a grating coupler (refer to, for example, JOURNAL OF QUANTUM ELEC IRONICS, vol. 38, no. 7, 2002, p. 949), which is an effective method that allows light to pass through without cutting out an Si optical circuit side.
However, the suggestion in Japanese Patent Application Laid-open No. H08-313756 has a problem in controllability of the tip width of the tapered waveguide. Namely, in a range where the spot size is reduced to 10 μm to about several μm, sensitivity of the spot size to the tip width is high, which causes a problem that manufacturing of a core layer with appropriate dimension accuracy is difficult.
Further, the suggestion in JOURNAL OF QUANTUM ELECTRONICS, vol. 38, no. 7, 2002, p. 949 has such a problem that fiber fixing processing is difficult, and the suggestion is unfit for integration and packaging.